Semiconductor laser chip

ABSTRACT

Semiconductor laser chip, whereby a laser-active semiconductor structure is arranged in an optical resonator. Two or more series-connected, laser-active pn-junctions whose forward directions are isodirected are introduced into one and the same resonator of the semiconductor laser chip. A respective pn-junction whose forward direction is opposite the forward direction of the laser-active pn-junctions is arranged between two laser-active pn-junctions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a semiconductor laser chip, wherebya laser-active semiconductor structure is arranged in an opticalresonator.

2. Description of the Relate Art

Such laser chips are disclosed, for example, by the publications U.S.Pat. No. 5,016,252, Japanese Published Application 2-71574 and JapanesePublished Application 61-247084. These disclose laser diode chipswherein an n-conductive cover layer, an active layer and a p-conductivecover layer are successively applied on a semiconductor substrate.

Semiconductor laser chips of this type are made of the material systemsInAlGaAs or InAlGaP on a GaAs substrate, InAlGaAsP on an InP substrateor InAlGaN on a sapphire or SiC substrate as edge which are referred toas vertical cavity surface emitting lasers (VCSEL).

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor laserchip that exhibits enhanced efficiency and an enhanced output powercompared to the Prior Art.

This object and others are achieved by semiconductor laser chips havinga laser-active semiconductor structure is arranged in an opticalresonator, two or more series-connected, laser-active pn-junctions whoseforward directions are isodirected introduced into the same resonator ofthe semiconductor laser chip, the laser-active pn-junctions containquantum wells for the charge carrier recombination, a pn-junction whoseforward direction is directed opposite the forward direction of thelaser-active pn-junctions is respectively arranged between twolaser-active pn-junctions, this reverse-poled pn-junction is located insemiconductor material having a larger band gap than that of thesemiconductor material of the laser-active pn-junctions, and thelaser-active pn-junctions are located in a wave-guiding zone.

In another embodiment, the emitting semiconductor laser chip has alaser-active semiconductor structure arranged in an optical resonator,two or more series-connected, laser-active pn-junctions whose forwarddirections are isodirected are introduced into one and the sameresonator of the semiconductor laser chip, the laser-active pn-junctionscontain quantum wells for the charge carrier recombination, apn-junction whose forward direction is directed opposite the forwarddirection of the laser-active pn-junctions is respectively arrangedbetween two laser-active pn-junctions, this reverse-poled pn-junction islocated in semiconductor material having a larger band gap than that ofthe semiconductor material of the laser-active pn-junctions, and thequantum wells of the laser-active pn-junctions are located in thebellies and the reverse-poled pn-junction is located in the node or,respectively, in the nodes of the standing wave field generated in thesemiconductor chip.

According to the invention two or more series-connected, laser-activepn-junctions are introduced into the same resonator of the semiconductorlaser chip, the forward directions of the pn-junctions beingisodirected. The laser-active pn-junctions contain quantum wells for thecharge carrier recombination. A pn-junction whose forward direction isopposite to the forward direction of the laser-active pn-junctions, thatis thus reverse-poled compared to the forward direction of thesemiconductor laser chip, is arranged between the pn-junctions. Thereverse-poled pn-junction or junctions are located in a semiconductormaterial having a greater band gap than the semiconductor material ofthe laser-active pn-junctions.

Given an edge-emitting semiconductor laser chip, the laser-activepn-junctions are located in a wave-guiding zone. Given verticallyemitting semiconductor laser chips (VCSELs), the quantum wells of thelaser-active pn-junctions lie in the bellies and the reverse-poledpn-junction or, respectively, the reverse-poled pn-junctions lie in thenode or nodes of the standing wave field generated in the semiconductorlaser chip.

Advantageously, the spacing between two successive laser-activepn-junctions amounts to less than 2 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

The quantum wells are undoped in a preferred embodiment.

The invention is explained in greater detail below on the basis of twoexemplary embodiments in conjunction with the Figures

FIG. 1A is a side cross-section of a top portion and FIG. 1B is a sidecross-section of a bottom portion of a laser diode according to thepresent invention;

FIGS. 2A and 2B are top and bottom portions of potential levels adjacentcorresponding to the laser diode structure;

FIGS. 3A and 3B are top and bottom portions of doping level diagramscorresponding to the diode structure; and

FIGS. 4A and 4B are top and bottom portions of a cross section of asecond embodiment of a laser diode of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The structure of an edge-emitting multiple pn laser diode shown in theFIGS. 1A and 1B comprises a substrate 1 that, for example, is composedof n-doped GaAs. An electrical contact 2, for example an n-contact thatis fabricated, for example, of GeNiAu, is applied onto a first principalsurface of this substrate 1. A first cladding layer 3 that, for example,is composed of AlGaAs, particularly of Al_(0.4)Ga_(0.6)As, comprises adoping of n≅5*10¹⁷ cm^(31')and a thickness of 1-2 μm is arranged on asecond principal surface of the substrate 1 lying opposite the firstprincipal surface.

An active layer sequence 4 is arranged on the first cladding layer 3,the sequence 4 containing two groups of preferably undoped quantumwells—specifically respectively two quantum wells 11 here—and beingenclosed by barrier layers 12 and capture layers 13-15. The quantumwells as are shown by the potential diagrams 2A and 2B, are composed,for example, of undoped InGaAs (for example, In_(0.2)Ga_(0.8)As quantumwells); the barrier layers 12 and capture layers 13-15 are composed, forexample, of GaAs or, respectively, Al_(0.2)Ga_(0.8)As.

The quantum wells 11 respectively lie in the depletion zones of pndiodes operated in a forward direction respectively composed of twoquantum wells 11, two barrier layers 12, an n-conductive and ap-conductive capture layer. The capture layer 14 arranged between thetwo groups of quantum wells 11 is fashioned as a highly doped diode (forexample, n≅5*10¹⁸ cm⁻³ and p≅5*10¹⁹ cm⁻³) which is reverse-poledrelative to the laser-active pn-junctions 8 and 9 and exhibits a greaterband gap than the laser-active pn-junctions. The doping levels areindicated by the doping diagrams FIGS. 3A and 3B.

A second cladding layer 5 is applied on the active layer sequence 4, thesecond cladding layer 5 being composed, for example, of AlGaAs,particularly of Al_(0.4)Ga_(0.6)As, comprising a doping p≅5*10¹⁷ cm⁻³and a thickness of 1-2 μm. A contact layer 6 on which an electricalcontact 7, a p-contact here, is applied is located on this claddinglayer 5. The contact layer 6 is composed, for example, of p⁺⁺-doped GaAsand is approximately 10 nm thick, and the p-contact 7 is composed, forexample, of TiPtAu.

The structure shown in FIGS. 1A and 1B can be directly manufactured asan edge-emitting broad-strip laser. It can also be employed inedge-emitting, transversally single-mode laser diodes that, for example,are to be realized in a ridge waveguide structure.

A further exemplary embodiment is a vertically emitting semiconductorlaser chip as shown in FIGS. 4A and 4B. Therein, Bragg reflector layers,for example AlAs—GaAs Bragg mirrors, are employed in a known way insteadof the two cladding layers 3 and 5 of the exemplary embodiment of FIGS.1A and 1B. In other words, the cladding layers 3 and 5 are replaced byreflectors, but otherwise the structure is the same.

The manufacture of the above-described layer sequences can, for example,ensue with molecular beam epitaxy. Carbon, for example, can serve forp-doping and silicon can serve for n-doping. The manufacture is alsopossible with metallo-organic vapor phase epitaxy.

The inventive structure of a semiconductor laser chip leads to anincrease in the gain to be achieved in the active layer sequence and,thus, to operation at lower threshold current densities. This enablesthe design of ultra-efficient large optical cavity structures.

The inventive structure is not limited to the InAlGaAs semiconductorsystem but can also be realized, for example, in the material systemsInAlGaAsP on InP substrate or, too, in the InAlGaNAs system. The laserstructure can also be realized in II-VI semiconductor systems such as,for example, ZnMgBeSSe.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventor to embody withinthe patent warranted hereon all changes and modifications as reasonablyand properly come within the scope of their contribution to the art.

What is claimed is:
 1. An edge-emitting semiconductor laser chip,comprising: an optical resonator; a laser-active semiconductor structurein said optical resonator; at least two series-connected laser-activepn-junctions in said optical resonator, said at least two laser-activepn-junctions having forward directions isodirected; said at least twolaser-active pn-junctions contain quantum wells for charge carrierrecombination; a reverse-poled pn-junction having a forward directiondirected opposite the forward direction of said at least twolaser-active pn-junctions and arranged between said at least twolaser-active pn-junctions, said reverse-poled pn-junction being locatedin semiconductor material having a larger band gap than that of thesemiconductor material of said at least two laser-active pn-junctions;and said at least two laser-active pn-junctions being located in awave-guiding zone.
 2. A vertically emitting semiconductor laser chip,comprising: an optical resonator; a laser-active semiconductor structurein said optical resonator; at least two series-connected laser-activepn-junctions in said optical resonator, said at least two laser-activepn-junctions having forward directions which are isodirected; said atleast two laser-active pn-junctions containing quantum wells for chargecarrier recombination; a reverse-poled pn-junction whose forwarddirection is directed opposite the forward direction of said at leasttwo laser-active pn-junctions and which is arranged between said atleast two laser-active pn-junctions; said reverse-poled pn-junctionbeing located in semiconductor material having a larger band gap thanthat of the semiconductor material of said at least two laser-activepn-junctions; and the quantum wells of said at least two laser-activepn-junctions being located in bellies and said reverse-poled pn-junctionbeing located in a node of a standing wave field generated in thesemiconductor chip.